1. Field of the Invention
This invention relates to an improved device and process for measuring the thickness of thermal film deposits on JFTOT tubes, and more particularly to such a device and process using a computer controlled interferometer.
2. Description of the Related Art
Jet fuels undergo chemical changes when subjected to thermal stress, changes that can affect the performance of these fuels. Modern aircraft designs place thermal stress on fuels from a number of sources, such as heating from the engines and frictional heating from the wing surfaces. Consequently, there is an ongoing effort to develop thermally stable jet fuels and to characterize the thermal stability of available jet fuels.
The Jet Fuel Thermal Oxidation Test (JFTOT) is widely used in this effort. In this test, the fuel is passed over a heated metal tube under conditions of limited oxygen availability. The thermal stability of the fuel is characterized by the quantity of insoluble reaction products formed. When this test is performed in accordance with ASTM D3241, the total quantity of insoluble products formed is determined by measuring the amount of insoluble products adhereing to the outside of the heated tube (determined by comparing the color of the tube deposits to standard color charts) and the amount of particulate insoluble products in the fuel (determined by measuring the pressure drop across an in-line filter during the test).
Most fuels fail on the basis of tube deposits. Unfortunately, most of the uncertainty in the test method is attributable to the uncertainty in the measurement of tube deposits. The standard color comparison charts have proven to be quantitatively unreliable.
To increase the reliability of the test, the tube deposit rater (TDR) method was developed to measure tube deposits more accurately. In this method, the attenuation of reflected white light, measured by a photocell, is correlated to the deposit thickness. TDR eliminates the subjectivity inherent in the color chart comparisons, but the method can be compromised by variations in the optical properties of the deposit. Neither TDR nor color chart comparisons correlate well with deposit carbon content determined by combustion.
Other interferometric devices that can measure the deposit thickness on the tube are known. These devices use manual positioning of the probe along the length of the tube. There are several inherent problems with these devices.
One problem with these manual devices is the inability of an operator to position the probe along the tube with accuracy and repeatability. Another problem is that these devices are not equipped for rotating the tube to perform scans about the entire circumference of the tube. Without means for rotating the tube, and without means for positioning the probe relative to the tube with accuracy and repeatability, it is impossible to get accurate information on the radial distribution of a deposit on the tube. As will be shown below, the thermal deposits on JFTOT tubes can show significant radial asymmetry.
Moreover, these devices, because they rely on the human hand to position the probe, may miss significant features of the longitudinal distribution of the deposit on the tube. As will be shown below, significant deposit thickness variations can be found within a very few millimeters along the length of the tube.
Another disadvantage of these manual devices is that they lack an automated system for identifying peaks in the light intensity data, and computing deposit thickness and volume from this data. This lack of automated control and analysis increases the time and tedium inherent in measuring thermal deposits on JFTOT tubes.
Darrah et al., U.S. Pat. No. 4,842,410, describes such a manual interferometer, and is incorporated by reference herein.